Hygienic absorbent products such as disposable diapers, feminine hygiene products and adult incontinence pads consist of a number of different layers, including a liquid permeable "topsheet" or "coverstock" layer which in use is located against the skin of the wearer, an absorbent core for retaining liquid and a liquid impermeable "backsheet" away from the skin of the wearer to maintain the absorbed liquid inside of the absorbent product. Additional layers are commonly provided with the aim of obtaining the desired characteristics in terms of acquisition and distribution of liquid, and the construction and function of such hygienic absorbent products is the subject of numerous patents and patent applications.
Although the characteristics of hygienic absorbent articles can vary depending upon the intended use, a few basic characteristics are generally sought, in particular the ability to absorb liquid at a sufficient rate and the ability to hold the absorbed liquid within the absorbent article, in addition to features such as comfort for the wearer and ease of production. Although such absorbent articles have undergone many advances in recent years, significant efforts continue to be made to improve their characteristics and to develop absorbent articles that are specially designed for specific uses. This has led to the development of a number of different composite nonwovens for the acquisition, distribution and/or storage of liquid in hygienic absorbent articles, some examples of which are the following:
EP 0 359 501-B1 discloses an absorbent structure containing cover, transfer and reservoir layers, each of which has preselected fluid retention and transfer capacity, in which the cover layer has a lower density than the transfer layer and tends to promote transfer of liquid to the transfer layer, and the transfer layer has a lower density than the reservoir layer and tends to promote transfer to the reservoir layer.
EP 0 532 005-A1 describes absorbent articles containing a backsheet layer, an absorbent body, an intermediate transfer layer and a bodyside liner layer, the bodyside liner layer being a bonded carded web composed of bicomponent fibres and having a basis weight of 15-40 g/m.sup.2.
EP 0 539 703-A1 discloses an absorbent article that includes, between a topsheet and a backsheet, an absorbent body containing a retention portion and, adjacent the topsheet, a surge management layer comprising bicomponent fibres and optionally also non-bicomponent fibres.
EP 0 596 191-A1 discloses a fibrous laminated material in which the two fibrous layers are bonded together in a bonding pattern with discrete bonding areas containing apertures, the first fibrous layer comprising fibres of one or more thermoplastic polymeric materials and the second fibrous layer comprising fibres of two or more thermoplastic polymeric materials.
FR 2 690 843-A1 discloses a process for manufacturing multilayer nonwovens in which a nonwoven web is formed on each of two different shaping cylinders, the webs then being joined together on a suction cylinder.
U.S. Pat. No. 4,892,534 discloses a nonwoven web for use as a bodyside liner, in particular for feminine pads, and containing at least 3 layers of thermoplastic filaments bonded together, the web being formed by spunbonding.
U.S. Pat. No. 5,300,054 discloses absorbent articles in which the absorbent core comprises pairs of layers, where each pair of layers comprises an acquisition/distribution layer and a storage layer, the storage layer of each pair of layers being located subjacent the acquisition/distribution layer.
WO 94/12713 discloses a multilayer nonwoven material comprising at least two layers of natural or synthetic textile fibres with progressively decreasing fibre denier from one layer to the next.
WO 94/14397 discloses an absorbent structure containing a first surge management layer of polymeric fibres and a second surge management layer of hydrophilic fibres, e.g. cellulosic fibres, with an absorbent layer including a high-absorbency material between the first and second surge management layers.
WO 94/22393 describes a topsheet having three separate layers, namely a first and second polymeric layer with an intermediate fibrous layer between the two polymeric layers, in which the fibrous material of the intermediate layer extends below the inner surface of the second polymeric layer, containing capillaries, e.g. in the form of apertures, that extend from a first surface of the topsheet to a second surface of the topsheet.
WO 94/28222 discloses a composite nonwoven containing at least two layers of carded fibres with progressively decreasing fibre denier from one layer to the next, the layers been joined together by needling.
WO 95/13776 describes absorbent articles containing, in addition to a topsheet, a backsheet and an absorbent core, an acquisition layer between the topsheet and the core, and optionally also an intermediate "liquid stable layer" with a structural support function between the acquisition layer and the core.
Despite the fact that many different composite nonwoven products are known and that many advances have been made in this field in recent years, prior art composite nonwovens for use in hygiene absorbent products suffer from certain disadvantages. One of these is the manner in which liquid is distributed in those nonwoven products having or functioning as an acquisition/distribution layer. Thus, although many nonwovens are designed to provide a certain distribution of absorbed liquid within the acquisition/distribution layer before the liquid is transferred to the absorbent core, this distribution is in general one which could be termed "passive distribution", meaning that liquid is absorbed and in most cases initially flows essentially directly to the absorbent core without any substantial distribution before being transferred to the core layer where liquid is stored. Such absorbent cores normally store liquid with the aid of a super absorbent polymer. These super absorbent polymers are extremely effective, since they are able to absorb many times their own weight of urine or other liquids, but upon wetting they form a gel which, while being effective to hold onto absorbed liquid, has the disadvantage that liquid is not easily transferred through such a gel agglomeration.
This results in a phenomenon known as "gel blocking", in which the first portion of liquid is readily absorbed, while subsequent portions must migrate around the gel area before they can be taken up by other parts of the absorbent core, with the possibility for leakage and retained liquid on the surface of the diaper. It is in this case that the prior art nonwovens distribute and transfer liquid by means of passive distribution, so that subsequent portions of liquid bypass the already wetted and gelled area of the core. With passive distribution, the first portion of liquid thus flows more or less directly to the absorbent core, and it is only subsequent portions that are directed--in a "passive manner" (because of the lack of access there where the core is already wetted)--to other portions of the core. It would thus be desirable to be have an acquisition/distribution layer with a more active distribution function, so that the liquid holding capacity of the absorbent core could be more effectively utilized.
A further problem in the prior art composite nonwovens and absorbent products is that the liquid absorbency rate is often limited by the topsheet. Thus, the "strike-through time" for known topsheets is at the best about 2.5 sec, and it would therefore be desirable to be able to improve the absorbency characteristics of absorbent articles by improving the strike-through time of the topsheet.
Another problem encountered is that of "rewet", i.e. leakage of absorbed liquid from the absorbent core. Prior art composite nonwovens typically have a rewet of not less than about 0.2 g (as determined by EDANA standard No. ERT 151.0-93) and it would be desirable to reduce the rewet value to below 0.2, and preferably as low as possible, thereby ensuring that absorbed liquid is maintained in the absorbent core.
With regard to the methods used for the production of nonwovens, including composite nonwovens, it is a general aim to be able to increase the production speed while at the same time maintaining sufficient strength and uniformity of the nonwovens. The problem arises, however, that an increase in production speed tends to lead to a decrease in strength and uniformity of the resulting nonwoven. In the case of composite nonwovens, it is important to be able to both produce individual layers having sufficient quality at a sufficient speed and to be able to assemble the composite nonwoven in an advantageous manner. A high-speed, in-line process resulting in composite nonwovens with the desired properties in terms of strength, uniformity and absorption characteristics would be particularly desirable. Related to this is the aim of providing a basic production process that is flexible enough to be suitable for producing a variety of different composite nonwovens in terms of varying acquisition and distribution patterns and in which the order of the layers can be adjusted as required.
The above problems associated with prior art composite nonwovens and methods for their production are addressed by the present invention.